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8 Fault Complexity

8.1 Definition

Fault complexity refers to the width and distribution of the deformed land around the fault trace.

Many faults appear to be simple, almost-straight linear features, on the ground surface with a narrow zone of deformation only a few metres wide. Sometimes they will take on the appearance on a terrace (such as that which can be seen along the Wellington fault at Totara Park, Upper Hutt) or a change in topographical alignment along a linear axis (such as along the Wairau fault in Marlborough where between 3 and 5 metres of lateral displacement can be seen).

Figure 8.1: Examples of simple linear fault features 

Other faults appear less well defined, and have a complex distributed zone of deformation. The Ostler fault (in the MacKenzie Basin near Twizel) takes the form of an irregular crease with meanders, variable changes in vertical displacement in a band of land deformation which varies greatly in width either side of the fault.

b. Wairau Fault

Wairau Fault. The most recent rupture along the well-defined trace of the Wairau section of the Alpine fault in Marlborough resulted in about 3–5 m of right lateral displacement at the fault (Lensen 1976, Zachariasen et al. 2001).

c.

The 1848 earthquake on the eastern section of the Awatere fault resulted in over 100 km of surface rupture along the fault, and as much as about 7 m of right-lateral displacement of the ground surface at the fault (Grapes et al. 1998, Benson et al. 2001).

Figure 8.2: Examples of complex deformation on the Ostler fault trace 

b.

These photos show the complex trace of the Ostler fault where surface rupture deformation, though concentrated at the fault, is also distributed over a relatively broad region on either side of the fault (Van Dissen et al. 1994). Arrows mark the location of surface fault rupture.

c.

Fault Complexity Classifications:

Faults can be grouped into a three-fold classification in regard to their complexity:

Class A: Well Defined: a fault trace of limited geographic width, typically a few metres to a few tens of metres wide.

Class B: Distributed: deformation is distributed over a broad geographic width up to hundreds of metres in width, sometimes with multiple fault traces, folds, or both.

Class C: Uncertain: The location of the fault trace(s) is uncertain as it either has not been mapped in detail or it cannot be identified. This is typically a result of gaps in the traces(s) (ie, the fault takes on the appearance of being broken up into segments), or erosion or coverage of the traces(s).

Figure 8.3: View of fault complexity types 

A fault trace is not always well defined. Faults may locally break into distributed segments, or there may be gaps along the fault trace, making location of the fault uncertain

Recent fault location studies (refer case studies section 12) have shown that certain faults can demonstrate all three levels of fault complexity at different parts of the fault. Variations on the three types of complexities discussed above may therefore be warranted.